Many polyurethane dispersions contain N-methylpyrrolidone (NMP), because it is often a required component in the polymerization process. In fact, NMP has been the solvent of choice for the widely used prepolymer mixing process (PMP) for aqueous polyurethane dispersion (PUD) synthesis. This particular solvent reduces the prepolymer viscosity during the production steps to enabling smooth handling and dispersing of the prepolymer. This is due to the good solubilizing characteristics of NMP for organic materials and its own water solubility. N-methylpyrrolidone also supports the coalescence of the dispersion particles when the dispersion is applied to a substrate, and thereby facilitates film formation.
Unfortunately, various studies have suggested that NMP may be a fetotoxin. Recent studies on NMP have persuaded the EU Commission Working Group on the Classification and Labeling of Dangerous Substances to label NMP as toxic. Formulations containing at least 5% NMP have to be labeled as being irritant and toxic “T”. In the United States, on Mar. 14, 2003, the State of California Office of Environmental Health Hazard Assessment (OEHHA) issued a Notice of Proposed Rulemaking to adopt regulatory levels for NMP listed pursuant to the Act as known to the State to cause reproductive toxicity (Title 22, California Code of Regulations, Section 12000). California Proposition 65 requires special labeling for products containing NMP.
In response to global regulations, a worldwide effort in the paint industry is being made to eliminate NMP from coating formulations. Alternative production processes have been developed to make aqueous polyurethane dispersions without NMP. These include methods to prepare these polymeric dispersions with alternate co-solvents such as ethylenically unsaturated monomers or polyurethane prepolymers that behave as reactive diluents, as well as with co-solvents such as acetone that can be removed from the product during processing. However, these approaches all have drawbacks such as poorer solubility of the polyurethane prepolymer, higher price, or limited availability. Some of the available co-solvents prevent the formation of a stable aqueous dispersion of the polyurethane polymer after preparation.
It would be desirable to provide an effective, low-cost process for preparing a stable polyurethane/acrylic polymer dispersion without the drawbacks of the prior art attempts.